Anodizing aluminum articles is well known. In this process, a protective oxide coating is formed at the surface of an aluminum article by electrolytic and/or chemical action. In the electrochemical process, aluminum articles are given a thin aluminum oxide coating by making the article anodic in an anodizing electrolyte such as sulfuric acid. Although the current applied to the anodizing cell may be AC or DC, usually a current having a DC component is applied such that the aluminum article is the anode.
Most prior art anodizing processes involve immersion of the workpiece in an electrolyte. These processes are time consuming and expensive to operate. When, for example, extrusions are anodized, complex electroconducting racks must be used to support the workpiece which is immersed in the anodizing bath. Heavy hoists and the like are required to manipulate these racks into and out of the bath. Immersion processes generally involve use of large tanks or vats containing anodizing solution. The use of these tanks requires venting of fumes emited from the extensive liquid surface areas. Disposal of the solution also creates economic and environmental problems.
In continuous prior art processes the strip is conveyed through vats or tanks of solution. These apparatus are complicated and energy intensive in addition to being relatively slow. For certain applications, where a high quality uniform, anodic film is required, as in lithographic plate, conventional tank type anodizing methods have proven less than desirable.
Prior art immersion type anodizing processes wherein the article is submerged in the anodic bath are somewhat difficult to control. As the oxide coating is generated, the resistivity of the cell increases changing the current density. Unless cell parameters are carefully monitored, uneven and non-uniform coatings are produced.
For lithographic applications, there is a requirement of a very uniform even anodized surface. Since anodized plate has a very smooth surface it must be treated by "graining" the surface. "Graining" creates a finish on the surface which substantially increases the surface area. Since the anodic coating is generated from the metal/oxide interface, the lithographic plate can be grained prior to anodizing.